This invention relates to organic electroluminescent devices that emit light from a current-conducting organic light emitting layer and have high luminance where the device employs a particular interlayer between a hole transport layer and the light emitting layer.
While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium electroluminescent element sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965, Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, xe2x80x9cDouble Injection Electroluminescence in Anthracenexe2x80x9d, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbons, were very thick (much greater than 1 xcexcm). Consequently, operating voltages were very high, often  greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g.  less than 1.0 xcexcm) between the anode and the cathode. Herein, the organic xe2x80x9cEL elementxe2x80x9d encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic EL element structure, described in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole transporting layer (HTL), and the other organic layer is specifically chosen to transport electrons, referred to as the electron transporting layer (ETL). The interface between the two layers provides an efficient site for the recombination of the injected hole/electron pair and the resultant electroluminescence.
There have been proposed organic EL elements that contain an organic light-emitting layer (LEL) between a hole-transporting layer and an electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer, a light-emitting layer and an electron transport/injection layer. These structures have resulted in improved device efficiency.
Materials comprising porphyrinic compounds have been disclosed by Tang in U.S. Pat. No. 4,356,429 for use in the HTL. Further improvements in device performance are taught in U.S. Pat. No. 4,539,507, U.S. Pat. No. 4,720,432, and U.S. Pat. No. 5,061,569 where the hole-transporting layer utilizes an aromatic tertiary amine.
Since these early inventions, further improvements in hole-transporting and other device materials have resulted in improved device performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. No. 5,061,569, U.S. Pat. No. 5,409,783, U.S. Pat. No. 5,554,450, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,908,581, U.S. Pat. No. 5,928,802, U.S. Pat. No. 6,020,078, and U.S. Pat. No. 6,208,077, amongst others. EP 891,121 and EP 1.029,909 suggest the use of biphenylene and phenylene diamine derivatives to improve hole injecting and/or transporting and JP 11-273830 suggests the use of naphthyldiamine derivatives in EL elements generally.
In JP 2001-118,683 and in JP 05-339,565 OLED devices have been suggested that include 3-phenylindolyl groups in the HTL or the LEL. Such materials have not been suggested in an interlayer between the HTL and the LEL.
Notwithstanding the foregoing developments, there are continuing needs for organic EL device components that, at a given current, will provide a desired high luminance with little or no increase in voltage necessary to obtain the luminance.
The invention provides a multilayer electroluminescent device comprising an anode, a hole transport layer (HTL), a light emitting layer (LEL), an electron transport layer (ETL), and a cathode, in that order, wherein there is located between the HTL and the LEL an interlayer (IL) comprising a compound containing a 3-phenylindolyl group. The invention also provides an imaging device containing such an electroluminescent device.
The device of the invention produces improved luminance with little or no increase in voltage necessary to obtain the luminance.